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Appendix VI.


of electric-welding processes. Introduced into England in 1902,,
thermit has been largely adopted for the welding together of the
ends of the rails of street tramways. Welded rails have been
always greatly desired, but the great difficulty was the question of
expansion and contraction due to the daily changes of tempera-
ture, which forbade the adoption of continuous rails on the
ordinary railway tracks. Rails in tunnels, however, and those
of street trams embedded in the non-conducting earth, are not
subject to such extreme temperature variations, and a forcible
holding down during those changes will only introduce a longi-
tudinal stress of between one and two tons per square inch, a
matter of no real consequence. The advantages of welded rails
are, on the other hand, great, both as regards smoothness of
working and electrical continuity. The welding was formerly
performed electrically, but it may be confidently asserted that
thermit welding has entirely superseded the older method, both
on account of its simplicity, and the great cheapness of first and
working costs.

Thermit is a granular mixture of aluminium and iron oxide, in
exact chemical proportions between the aluminium and the
oxygen; so that a perfect union may ensue on combustion, and
nothing but alumina and pure iron remain behind as products.
Its temperature of ignition is a little less than that of molten steel,
and below this it will not ignite. It is perfectly safe, therefore,
against a red-hot poker, or even molten cast iron; and may be
thrown into an ordinary fire without injury. It would be ignited,
however, by molten steel, or, indeed, by any other method which
would produce the temperature of ignition, however small the
area of attack. In practice a special ignition powder is provided,
which may be started by a lucifer match, thus causing a local
heat of sufficient intensity to ignite the thermit, whose ingredients
now combine and cause the enormous temperature of 5400 F. by
their combustion. The iron being freed, the aluminium goes to
form with the oxygen a slag of alumina, which appears as thin,
dark red flakes of what may be called emery, ruby, or sapphire,
both slag and iron remaining fluid at the high temperature. It is
this immense evolution of heat that serves fpr the welding of two
pieces of iron or steel, being thus analogous to electric welding,